The present invention generally pertains to video communication systems and is particularly directed to a system useful in subscription television wherein the video signal is scrambled to prevent unauthorized use of the video signal by non-subscribers.
It is common practice to scramble a video signal by altering the content or shape of the video signal to prevent a stabilized picture from being displayed by a typical broadcast television set receiving the scrambled video picture. One of the scrambling techniques employed is the alteration of the synchronizing information in the video signal. The scrambled signal is decoded by a decoder connected to the subscriber's television set. The decoder is designed to convert the scrambled signal to enable a stabilized picture to be displayed on the subscriber's typical broadcast television set. Some decoders are enabled only when they are equipped with a "ticket" module containing electronic circuitry essential to the decoding operation, or for recognizing a coded signal that is transmitted with or as a part of the scrambled video signal provided by the encoder. The coded signals and the ticket modules are changed periodically as subscription periods expire and are renewed.